% Dinosaur and % Prehistoric refers to the percentage of random encounters (and presumably placed lairs) that consist of Dinosaurs (including pterodactyls, plesiosaurs, dimetrodons and the like) or "Prehistoric" encounters (such as mammoths, smilodons, cave men, axe beaks, hobbits, etc.) in the noted geographic area, with a roll being made on the appropriate tables for the composition of said encounter, such as the tables in the 1st Edition Dungeon Masters Guide.

% Lovecraft would be an additional modifier for any of the preceeding; hence "35% Dinosaur (5% Lovecraft)" would give you the percentage chance that any given dinosaur encountered may be squamous, polypous or be festooned with abundant facial feelers or tentacular growths. Lovecraftian entities would also have extra spells and unusual abilities relating to transits through, or unusual relationships with, time and space.

@ Johnstone, Needles & Huth: Apparently this thought experiment has uncovered a previously unnoticed glaring need for combination encounters that combine all of the different encounter elements into one creature...

@ B.Portly: I think that comment was intended for the MAR Barker post? Regardless, AD&D Dinosaurs can very well be a recipe for a TPK! Sort of like much of King King...

I'd say you should take a look at "The Purple Pterodactyls" (The collection, not just that story). I think that it could provide some interesting thoughts. I can't personally speak for any of his other books, as I haven;t read them, but they should also be good.